Multi-layer optical disk and method of producing multi-layer optical disk

ABSTRACT

A multilayer optical disk including a plurality of information storage layers to/from which information is recorded and/or reproduced by an optical head. The plurality of information storage layers are stacked through intermediate layers and each information storage layer includes a wobbling track groove. The shape factors (amplitude of wobbling, depth of the track groove and/or slope angle of the side wall of the track groove, etc.) of the track groove of at least one information storage layer of the plurality of information storage layers are different from the shape factors of track grooves of other information storage layers. This allows a satisfactory signal to be reproduced from each information storage layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Section 371 of International Application No.PCT/JP01/08477, filed Sep. 27, 2001, the disclosure of which isincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an optical storage medium such as anoptical disk and a method for producing the optical storage medium, andmore particularly, to a multilayer optical disk comprising a pluralityof information storage layers with guide grooves (track grooves).

BACKGROUND ART

With the widespread use of compact disks (CD), the optical disk has wona position of an important storage medium. Readable/writable disks, suchas is CD-R and CD-RW disks, which can not only reproduce but also recordinformation is also widely used. Research and development of opticaldisks of higher density is at full blast in recent years.

In increasing the recording density of an optical disk, not onlyincreasing the recording density of one information storage layer butalso increasing the number of information storage layers is effective.In DVD family, there is a read-only optical disk that allows informationrecorded on two information storage layers to be read from one side. Inaddition to the read-only optical disk having two information storagelayers, a readable/writable optical disk having two information storagelayers is also under development.

With reference to FIG. 1, a configuration of a readable/writable opticaldisk having two information storage layers will be explained.

The readable/writable optical disk shown in FIG. 1 has two informationstorage layers made of a phase change material whose opticalcharacteristic changes between amorphous and crystalline phases. On eachinformation storage layer, an amorphous pattern called “mark” isrecorded by irradiation with a laser beam.

The optical disk in FIG. 1 comprises a first substantially transparentsubstrate 201 having a track groove (groove) and a second substantiallytransparent substrate 205 having a track groove which are bonded to eachother. A semi-transparent first information storage layer 202 is formedon the first substantially transparent substrate 201, and a secondinformation storage layer 204 is formed on the second substantiallytransparent substrate 205. Both substrates 201 and 205 are placed insuch a way that the two information storage layers 202 and 203 face eachother, and are bonded to each other by means of a substantiallytransparent bonding layer 203. The bonding layer 203 functions as anintermediate layer that separates the first information storage layer202 from the second information storage layer 203.

The track grooves of the respective information storage layers 202 and204 wobbles at a predetermined frequency. When information isrecorded/reproduced, a readout signal having the frequency is detectedand a clock signal is generated. The clock signal is used to adjust therotation speed of the disk with the read/write speed of the diskapparatus.

In such a multilayer optical disk, compared to the amount of lightincident from the optical head of the disk apparatus upon the opticaldisk, the amount of light that returns from each information storagelayer to the photo-detection area of the optical head is quite small.This causes the readout signal obtained from the wobbling of the trackgrooves on the respective storage layers to become small.

Furthermore, since the structure of the recording film (lighttransmittance and reflectance, etc.) differs from one informationstorage layer to another, the ratio of the amplitude of a readout signalto the noise level of the readout signal (CN ratio: Carrier to NoiseRatio) may vary considerably among a plurality of information storagelayers. In this case, it is difficult to reproduce the clock signalaccurately based on the wobbling shapes of the track grooves of therespective information storage layers.

It is an object of the present invention to provide a multilayer opticaldisk capable of reliably reproducing a clock signal based on wobblingshapes of track grooves of respective information storage layers.

DISCLOSURE OF INVENTION

The multilayer optical disk according to the present invention is amultilayer optical disk including a plurality of information storagelayers to/from which information is recorded and/or reproduced by anoptical head, wherein the plurality of information storage layers arestacked through intermediate layers, each information storage layerincludes a wobbling track groove, and shape factors of the track grooveof at least one information storage layer of the plurality ofinformation storage layers are different from the shape factors of trackgrooves of other information storage layers.

In a preferable embodiment, the shape factors of the track grooveinclude a wobbling amplitude of the track groove along the radialdirection of the disk, a depth of the track groove and/or a slope angleof the side wall of the track groove.

In another preferable embodiment, a different value is given to any oneof the shape factors of the track groove for each information storagelayer and the amplitude of a readout signal caused by the wobbling ofthe track groove is thereby adjusted.

In a further preferable embodiment, the shape factors of the trackgroove are adjusted in such a way that the CN ratios related to theamplitudes of the readout signals caused by the wobbling of the trackgrooves of the plurality of information storage layers havesubstantially the same values and variations in the CN ratios areadjusted to within 30% among the information storage layers.

In a further preferable embodiment, the wobbling of the track groovecontains a basic frequency component, which oscillates in almost singlecycle used for reproduction of a clock signal.

In a further preferable embodiment, the wobbling of the track grooveexhibits a shape that varies according to sub-information and contains ahigher frequency component than the basic frequency component.

In a further preferable embodiment, the sub-information containspositional information indicating addresses on the disk.

In a further preferable embodiment, the wobbling shape of the trackgroove includes a combination of a sine wave and/or generallyrectangular waveform.

In a further preferable embodiment, the wobbling amplitude of therectangular waveform is set to be greater than the wobbling amplitude ofthe sine wave.

The method for producing a multilayer optical disk according to thepresent invention is a method for producing a multilayer optical diskincluding a plurality of information storage layers comprising amastering step of preparing a plurality of metal dies and a replicatingstep of producing a substrate onto which a desired pattern istransferred using the plurality of metal dies and formingrecordable/reproducible information storage layers, wherein in themastering step, a plurality of metal dies for the plurality ofinformation storage layers having a shape that specifies a track groovewhose at least one shape factor differs from one information storagelayer to another are produced.

In a preferable embodiment, the mastering step includes a step ofpreparing a plurality of substrates to which a photosensitive materialis applied, a recording step of forming a latent image of a patternincluding the wobbling track groove by irradiating a selected area ofthe photosensitive material with recording light, a developing step ofproducing a plurality of master disks having the above-described patternby developing the photosensitive material, and a step of producing aplurality of metal dies based on the plurality of master disks, whereinin the recording step, the amount of deflection of the recording lightalong the radial direction of the disk is changed for each substrate andthe amplitude of wobbling of the track groove is thereby changed foreach information storage layer.

In a preferable embodiment, the mastering step includes a step ofpreparing a plurality of substrates to which a photosensitive materialis applied, a recording step of forming a latent image of a patternincluding the wobbling track groove by irradiating a selected area ofthe photosensitive material with recording light, a developing step ofproducing a plurality of master disks having the above-described patternby developing the photosensitive material, and a step of producing aplurality of metal dies based on the plurality of master disks, whereinin the recording step, the thickness of photosensitive material ischanged for each of the plurality of metal dies.

In a preferable embodiment, the mastering step includes a step ofpreparing a plurality of substrates to which a photosensitive materialis applied, a recording step of forming a latent image of a patternincluding the wobbling track groove by irradiating a selected area ofthe photosensitive material with recording light, a developing step ofproducing a plurality of master disks having the above-described patternby developing the photosensitive material, and a step of producing theplurality of metal dies based on the plurality of master disks, whereinthe slope angle of the side wall of the track groove is changed for eachof the plurality of metal dies.

In a preferable embodiment, the slope angle of the side wall of thetrack groove is changed by applying heating processing to the masterdisk after the developing step.

In a preferable embodiment, the slope angle of the side wall of thetrack groove is changed by applying plasma processing to the metal dieafter the mastering step and before the replicating step.

In a preferable embodiment, argon and/or oxygen is used for the plasmaprocessing.

In a preferable embodiment, the recording light is deflected accordingto a pattern combining a sine waveform and rectangular waveform in therecording step.

In a preferable embodiment, the amount of deflection of the recordinglight is changed between the sine waveform section and the rectangularwaveform section.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view illustrating a structure of a conventionaloptical disk;

FIG. 2 is a schematic view illustrating a track groove of a multilayeroptical disk according to the present invention;

FIG. 3 illustrates details of the above-described track groove;

FIG. 4 is a cross-sectional view showing a structure of a firstembodiment of a multilayer optical disk according to the presentinvention;

FIG. 5 is a plan view illustrating wobbling patterns of track grooves ofthe first embodiment;

FIG. 6 is a perspective view illustrating wobbling patterns of the trackgroove of the first embodiment;

FIG. 7 is a process sectional view illustrating a method for producingthe multilayer optical disk according to the first embodiment;

FIG. 8 is a process sectional view illustrating a first replicating stepaccording to the first embodiment;

FIG. 9 is a process sectional view illustrating a second replicatingstep according to the first embodiment;

FIG. 10 is a cross-sectional view showing a structure of a secondembodiment of a multilayer optical disk according to the presentinvention;

FIG. 11 is a graph illustrating a relationship between a depth of atrack groove and a signal amplitude;

FIG. 12 is a cross-sectional view showing a structure of a thirdembodiment of a multilayer optical disk according to the presentinvention;

FIG. 13 is a plan view showing another example of track grooves;

FIG. 14( a) is a plan view showing wobble pattern elements;

FIG. 14( b) is plan view illustrating 4 types of wobble patterns formedby combining the above-described elements;

FIG. 15 illustrates a basic configuration of an apparatus capable ofidentifying the type of a wobble pattern based on a wobble signal whoseamplitude changes according to the wobbling of a track groove;

FIG. 16 is a waveform diagram showing a wobble pattern, wobble signaland pulse signal of a track groove; and

FIG. 17 illustrates a circuit configuration that separates a pulsesignal and clock signal from a wobble signal.

BEST MODE FOR CARRYING OUT THE INVENTION

The multilayer optical disk according to the present invention canaccurately reproduce a signal based on the wobbling of a track groove bychanging shape parameters (shape factors) of the track groove for eachinformation storage layer.

With reference now to the attached drawings, a configuration of a trackgroove of the optical disk will be explained in detail below.

On a recording plane 1 of the optical disk medium according to thepresent invention, a track groove 2 is formed in a spiral shape as shownin FIG. 2. FIG. 3 shows an enlarged view of part of the track groove 2.In FIG. 3, a disk center (not shown) exists below the track groove 2 anda disk radial direction is indicated by the arrow a. The arrow b pointsa direction in which a read/write light beam spot, being formed on thedisk, moves as the disk is rotated. In the following description, adirection parallel to the arrow a will be herein referred to as a “diskradial direction” (or “radial direction” simply), while a directionparallel to the arrow b will be herein referred to as a “trackingdirection”.

In a coordinate system in which the light beam spot is supposed to beformed at a fixed position on the disk, a part of the disk irradiatedwith the light beam (which will be herein referred to as a “diskirradiated part”) moves in the direction opposite to the arrow b.

Hereinafter, the X-Y coordinate system illustrated in FIG. 3 will beconsidered. In an optical disk according to the present invention, the Ycoordinate of a position on a side face 2 a or 2 b of the track groovechanges periodically as the X coordinate thereof increases. Such aperiodic positional displacement on the groove side face 2 a or 2 b willbe herein referred to as the “wobble” or “wobbling” of the track groove2. A displacement in the direction pointed by the arrow a will be hereinreferred to as a “radially-outwardly displacement”, while a displacementin the direction opposite to the arrow a will be herein referred to as a“radially-inwardly displacement”. Also, in the Figure, one wobble periodis identified by “T”. The wobble frequency is inversely proportional toone wobble period T and is proportional to the linear velocity of thelight beam spot on the disk.

In the illustrated example, the width of the track groove 2 is constantin the tracking direction (as indicated by the arrow b). Accordingly,the amount to which a position on the side face 2 a or 2 b of the trackgroove 2 is displaced in the disk radial direction (as indicated by thearrow a) is equal to the amount to which a corresponding position on thecenterline of the track groove 2 (as indicated by the dashed line) isdisplaced in the disk radial direction. For this reason, thedisplacement of a position on the side face of the track groove in thedisk radial direction will be herein simply referred to as the“displacement of the track groove” or the “wobble of the track groove”.It should be noted, however, that the present invention is not limitedto this particular situation where the centerline and the side faces 2 aand 2 b of the track groove 2 wobble to the same amount in the diskradial direction. Alternatively, the width of the track groove 2 maychange in the tracking direction. Or the centerline of the track groove2 may not wobble but only the side faces of the track groove may wobble.

The optical disk of the present invention comprises a plurality ofinformation storage layers and the above-described wobbling track grooveis formed on each information storage layer. A main feature of themultilayer disk of the present invention is that shape factors of atrack groove are not uniform but different from one information storagelayer to another.

Hereunder, embodiments in which three types of shape parameters of atrack groove are adjusted will be explained more specifically.

Embodiment 1

First, with reference to FIGS. 4 and 5, a first embodiment of theoptical disk of the present invention will be explained.

As shown in FIG. 4, the multilayer optical disk according to thisembodiment is a multilayer optical disk including a plurality ofinformation storage layers to/from which information is recorded and/orreproduced by an optical head. The plurality of information storagelayers are stacked through intermediate layers and each informationstorage layer has a surface with a wobbling track groove and a storagelayer. FIG. 5 shows how track grooves wobble. The width of the trackgroove is approximately 0.10 to 0.25 μm and the depth is approximately10 to 25 nm.

FIG. 4 will be referenced again. More specifically, the optical disk ofthis embodiment comprises a polycarbonate sheet (80 μm thick) 301, afirst UV cure resin layer (10 μm thick) 302, a first semitransparentinformation storage layer (10 μm thick) 303, a second UV cure resinlayer (20 to 40 μm thick) 306, a second information storage layer 304and a polycarbonate substrate 305, in the order from the side on whichthe optical head is placed.

The first UV cure resin layer 302 has track grooves with approximately0.32 μm track pitches and pits formed in the inner area. Pit locationsrepresent non-rewritable information.

Both the first information storage layer 303 and second informationstorage layer 304 contain a phase change recording materials mainlycomposed of GeTeSb.

The second UV cure resin layer 306 bonds the first information storagelayer 303 and second information storage layer 304, and also functionsas an intermediate layer that separates the two information storagelayers.

On a first plane of the polycarbonate substrate 305, track grooves withapproximately 0.32 μm pitches are formed spirally or concentrically andnon-rewritable pits are provided in the inner area of the disk.

Recording/reproduction of the second information storage layer 304 byirradiation with a laser beam is conducted through the first informationstorage layer 303. Thus, the first information storage layer 303 has atransmittance of approximately 50% with respect to the laser beam usedfor recording/reproduction.

The various feature sizes such as thickness of each layer, width anddepth of each track groove do not reflect their actual sizes. Forexample, the depth of the track groove is no more than a fraction of awavelength of the laser beam used for recording/reproduction, while thethickness of the second UV cure resin layer 306 (that is, distancebetween the upper and lower information storage layers) ranges fromseveral tens of times to even 100 times the above-described wavelength.

A feature of the optical disk of this embodiment is that the amplitudeof wobbling of a track groove of the first information storage layer 303is different from the amplitude of wobbling of a track groove of thesecond information storage layer 304. This point will be explained inmore detail below.

The track grooves of the respective information storage layers 303 and304 wobbles in an almost sine waveform at a single frequency as shown inFIG. 5. A frequency of a clock signal is defined based on thisfrequency. That is, the wobbling frequency of a track groove representsclock information. Here, the “amplitude of wobbling” of the track grooverefers to the amplitude of wobbling measured along the disk radialdirection.

The above-described clock information is reproduced by the optical headwith a numerical aperture of 0.85, which emits a laser beam with awavelength of 405 nm. More specifically, the laser beam reflected fromthe optical disk is detected by a photo-detection area divided into twoportions to the right and left with respect to the track direction toproduce a difference between the two signals (push-pull signal). Thispush-pull signal is used to control an optical pickup in such a way thata laser beam spot keeps track of the track groove. The push-pull signalincludes a frequency component that follows the wobbling of the trackgroove, but the frequency band of the wobbling is higher than thefrequency band of the signal component, which is important in trackingcontrol. Thus, applying appropriate filtering to the push-pull signalmakes it possible to separate/detect the clock information. It ispossible to provide a portion where the track groove breaks at someregion of the track groove to record information other than the clockinformation in that portion.

In this embodiment, as shown in FIG. 6, the two information storagelayers have identical pitches of track grooves (track pitches), but theamplitude of wobbling W1 of the track groove of the first informationstorage layer 303 and the amplitude of wobbling W2 of the track grooveof the second information storage layer 304 are different.

As a comparative example, assuming that the amounts of wobbling of therespective information storage layers W1 and W2 are equivalent toapproximately 4% of the pitch of the track groove, the ratio of theintensity of the reflected light which is reflected by the firstinformation storage layer 303 and detected by the photo-detection areaof the optical head to the intensity of the laser light incident fromthe optical head was approximately 7%, while the ratio of the intensityof the reflected light which is reflected by the second informationstorage layer 304 and detected by the photo-detection area of theoptical head was approximately 5%.

Furthermore, when data in the second information storage layer 304 isrecorded/reproduced, since the first information storage layer 303exists in the path of the laser beam, an optical signal reflected by thesecond information storage layer 304 includes noise caused by thepresence of the first information storage layer 303.

Thus, when the intensity of the reflected light from the secondinformation storage layer 304 decreases and noise increases, the CNratio of the reproduced signal decreases. Furthermore, reading the clockinformation is based on the wobbling of the track groove satisfactorilyrequires a CN ratio of 30 dB or more. A relationship between theamplitude of wobbling of the track groove W and the CN ratio is shown inTable 1.

TABLE 1 Amount of wobbling Amount of wobbling 10 nm 15 nm 1stinformation storage 33 dB — layer (W1) 2nd information storage 28 dB 33dB layer (W2)

As is appreciated from Table 1, when both the amplitude of wobbling ofthe track groove W2 of the second information storage layer 304 and theamplitude of wobbling of wobbling of the track groove W1 of the firstinformation storage layer 303 were equally set to 10 nm, the CN ratio ofthe signal reproduced from the second information storage layer 304 fellbelow 30 dB. However, when the amplitude of wobbling W1 was set to 10 nmand the amplitude of wobbling W2 was set to 15 nm, the CN ratio of 33 dBwas obtained for the signals reproduced from both information storagelayers.

If there is a considerable difference in the amount of light reflectedby the respective information storage layers and detected by thephoto-detection area of the optical head depending on the informationstorage layers, a problem occurs when the focal point of the opticalhead is moved between the two information storage layers whenreading/writing data to/from different information storage layers. Toavoid this problem, it is desirable to adjust the amount of the detectedlight in such a way that the amount of light with respect to theinformation storage layer where the amount of the detected light reachesa maximum is not more than twice the amount of the light with respect tothe information storage layer where the amount of the detected lightreaches a minimum.

It is preferable to adjust a variation in the CN ratio with respect tothe amplitude of the reproduced signal caused by the wobbling of thetrack groove to within 30% between the information storage layers.

Then, the method of producing the multilayer optical disk of thisembodiment will be explained with reference to FIG. 7.

First, a first glass substrate (thickness: for example, approximately 6mm) 501 and a second glass substrate (thickness: for example,approximately 6 mm) 502 are cleaned. Then, a first master disk 503 witha photoresist (thickness: approximately 10 to 40 mm) applied onto thefirst glass substrate 501 and a second master disk 504 with aphotoresist (thickness: approximately 10 to 40 mm) applied onto thesecond glass substrate 502 are prepared. The photoresists formed on theglass substrates 501 and 502 have substantially the same thickness.

Then, recording/developing steps of transferring predetermined patternsto the two master disks 503 and 504 are conducted. More specifically, alaser beam with a wavelength of 248 nm is focused on the photoresist forexposure. While rotating the master disks 503 and 504, the position ofthe beam spot of the laser light on the photoresist is displaced in aradial direction of the disk. This periodical displacement is producedby deflecting the laser beam. In this way, a pattern of a wobbling trackgroove is transferred to the photoresist. By the way, modulating theintensity of the laser beam makes it possible not only to stop theformation of the track groove but also to control a physical shape suchas the width of the track groove. Thus, a desired pattern including thetrack groove is transferred to the photoresist as a latent image. Then,the predetermined pattern is given to the photoresist after developmentand a first master disk 505 and second master disk 506 are produced.

A Ni thin film is deposited on the master disks 505 and 506 using asputtering method. Then, Ni electroforming is performed using the Nithin film as an electrode and a Ni layer of approximately 300 μm inthickness is formed. After removing the Ni layer from the master disks505 and 506, the photoresist adhered to the Ni layer is removed and theback of the Ni layer is polished. Unnecessary parts that define theinner diameter and outer diameter of the disk are punched out from thisNi layer and a first stamper 507 and a second stamper 508 that functionas metal dies of the optical disk are produced (mastering step).

Then, a first substrate 509 on which a first information storage layeris formed is produced using the first stamper 507. This step (firstreplicating step) will be explained with reference to FIG. 8.

First, a polycarbonate substrate master 601 will be produced byinjection molding using the first stamper 507. A concavo-convex patternof the first stamper 507 is transferred to the surface of the substratemaster 601. An aluminum film is deposited on the pattern transferredsurface of the substrate master 601 using the sputtering method.

On the other hand, a circular sheet 602 made of a polycarbonate sheet ofapproximately 80 μm in thickness is prepared and UV cure resin isdischarged onto this circular sheet 602 in a doughnut shape.

Then, the circular sheet 602 is laid on the substrate master 601 withthe surface on which the aluminum film is formed facing the circularsheet 602. By rotating the substrate master 601, the extra UV cure resinis removed by a centrifugal force. Thus, the thickness of the UV cureresin between the substrate master 601 and the sheet 602 is adjusted toapproximately 10 μm.

After hardening the UV cure resin by irradiation with ultraviolet rays,the hardened UV cure resin and the sheet 602 are removed from thesubstrate master 601. The UV cure resin and the sheet film 602 arebonded to constitute a sheet substrate 603. On the surface of this sheetsubstrate 603, the pattern of the first stamper 507 is transferred.

On the pattern transferred surface of the sheet substrate 603, a firstdielectric film (thickness: approximately 50 to 1000 nm) 604, arecording film (thickness: approximately 3 to 50 nm) 605, a seconddielectric film (thickness: approximately 50 to 1000 nm) 606 and a metalreflecting film (thickness: approximately 0 to 40 nm) 607 are stacked inthat order. The metal reflecting film 607 can be omitted. Both the firstdielectric film 604 and second dielectric film 606 are made of amaterial predominantly composed of ZnS and the recording film 605 isformed of a phase change recording material predominantly composed ofGeTeSb. The metal reflecting film 607 is made of an Ag alloy film andhas a thickness, which makes it semi-transparent to the laser beam usedfor recording/reproduction. All the layers that constitute thismultilayer (information recording film) are preferably deposited using asputtering method.

The recording film 605 formed using the sputtering method is in anamorphous state immediately after film formation. To initialize therecording film 605, a laser beam is focused and incident upon therecording film 605 to crystallize the recording film 605. The firstsubstrate 509 is produced in this way.

Then, a second substrate 510 will be produced using the above-describedsecond stamper 508. This step (second replicating step) will beexplained with reference to FIG. 9.

First, a polycarbonate substrate master 701 of approximately 1.1 mm inthickness will be produced by injection molding using the second stamper508. A concavo-convex pattern of the second stamper 508 is transferredto the surface of the substrate master 701.

On the pattern transferred surface of the substrate master 701, a metalreflecting film 705, a second dielectric film 704, a recording film 703and a first dielectric film 702 are stacked in that order. Thesemultilayers (information recording films) are preferably formed using asputtering method.

The metal reflecting film 705 is made of a metal film predominantlycomposed of aluminum and the first dielectric film 702 and the seconddielectric film 704 are formed of a film predominantly composed of ZnS.The recording film 703 is formed of a phase change recording materialmainly composed of GeTeSb. All these layers constituting the multilayer(information recording film) are preferably deposited using thesputtering method.

As in the case with the first substrate, a laser beam is condensed andirradiated onto the recording film 703 to crystallize and initialize therecording film 703. The second substrate 510 is produced in this way.

FIG. 7 will be referenced again.

UV cure resin is discharged concentrically onto the surface on which theinformation storage layer of the first substrate 509 is formed. Then,the substrate with the first information storage layer is laid on thesubstrate 510 with the second information storage layer in such a waythat the surface on which the information storage layer of the substrate510 is formed faces the substrate with the first information storagelayer. By rotating these substrates and shaking off the extra UV cureresin by a centrifugal force, the thickness of the UV cure resin isadjusted to approximately 20 to 40 μm.

By hardening the UV cure resin by irradiation with ultraviolet rays,both substrates are bonded together and an intermediate layer forseparating both information storage layers is formed. The multilayeroptical disk 511 having two information storage layers is formed in thisway.

By the way, the figure shows that the position of the track groove ofthe upper information storage layer perfectly aligns with the positionof the track groove of the lower information storage layer, but this isnot required in practice. Since tracking control is performed for eachinformation storage layer, the positions of track grooves need not havea specific relationship with each other among different informationstorage layers.

This embodiment requires that the amplitude of wobbling of a trackgroove should differ from one information storage layer to another.Thus, the amount of deflection of the laser beam is differentiatedbetween the step of recording a track groove pattern on the photoresiston the first master disk 505 and the step of recording a track groovepattern on the photoresist on the second master disk 506. Morespecifically, the amount of deflection of the laser beam is adjusted forthe first master disk 505 so that the amplitude of wobbling shown inFIG. 6 becomes W1. In contrast, the amount of deflection of the laserbeam is adjusted for the second master disk 506 so that the amplitude ofwobbling shown in FIG. 1 becomes W2 (≠W1). This gives the firstsubstrate 509 with the amplitude of wobbling of the track grooveadjusted to W1 and the second substrate 510 with the amplitude ofwobbling of the track groove adjusted to W2.

Embodiment 2

With reference to FIG. 10, a second embodiment of the multilayer opticaldisk according to the present invention will be explained.

The multilayer structure of the multilayer optical disk according tothis embodiment is substantially identical to that of the multilayeroptical disk according to the first embodiment. However, in the case ofthe optical disk of this embodiment, the depth D1 of a track groove of afirst information storage layer 801 is different from the depth D2 of atrack groove of a second information storage layer 802.

FIG. 11 is a graph showing a relationship between the amplitude of areadout signal and the depth of the track groove originated from thewobbling of the track groove assuming that the wavelength of a laserbeam emitted from the optical head is λ. The depth of the track grooveis converted to an optical path length. As is apparent from the graph inFIG. 11, the amplitude of the readout signal reaches a maximum when thedepth of the track groove is λ/8 and the amplitude of the readout signaldecreases as the depth approximates to λ/4. The depth D1 of the trackgroove of the first information storage layer and the depth D2 of thetrack groove of the second information storage layer are preferably setto λ/8 or less.

In the case of a multilayer optical disk, there is a tendency that noiseincluded in the readout signal from the second information storage layer802 becomes greater than noise included in the readout signal from thefirst information storage layer 801. Assuming that the depth of thetrack groove D1=the depth D2 of the track groove, the ratio of intensityof the laser beam reflected by the first information storage layer 801and detected by the photo-detection area to the intensity of theincident laser beam output from the optical head is approximately 7%,whereas the proportion of intensity of the laser beam reflected by thesecond information storage layer 802 and detected by the photo-detectionarea is approximately 5%.

A CN ratio when the optical head with a numerical aperture of 0.85 thatemits a laser beam with a wavelength of 405 nm is used to performrecording/reproduction is shown in Table 2. The required CN ratio is 30dB or greater.

TABLE 2 Depth 16 nm Depth 18 nm 1st information storage 33 dB — layer(D1) 2nd information storage 27 dB 31 dB layer (D2)

As is apparent from Table 2, by making the depth D2 of the track grooveof the second information storage layer 802 by approximately 10 to 20%greater than the depth D1 of the track groove of the first informationstorage layer 801, almost the same CN ratio of 30 dB or more wasobtained for both information storage layers. When the amount of lightreflected by the information storage layer and detected by thephoto-detection area of the optical head varies considerably dependingon the information storage layer, a problem occurs when the focus of theoptical head is moved between the two information storage layers whenrecording/reproduction is performed on different information storagelayers. To avoid this problem, it is preferable to adjust the amount oflight corresponding to the information storage layer whose amount oflight detected becomes a maximum to not more than two times the amountof light corresponding to the information storage layer whose amount oflight detected becomes a minimum.

Then, a method of producing a multilayer optical disk according to thisembodiment will be explained. This method of producing a multilayeroptical disk is substantially the same as the method of producing amultilayer optical disk according to the first embodiment. Thisembodiment is different in that the thicknesses of photoresists formedon the first master disk and second master disk are set to D1 and D2(≠D1), respectively.

By the way, it is also possible to change the depth of the track groovefor each information storage layer by making the thickness of thephotoresist applied to the respective master disks greater than thedepth of a desired track groove and changing the intensity of the laserbeam in the exposure step between the two master disks.

Embodiment 3

With reference to FIG. 12, a third embodiment of the multilayer opticaldisk according to the present invention will be explained.

The multilayer structure of the multilayer optical disk according tothis embodiment is substantially the same as that of the multilayeroptical disk according to the first embodiment. However, in the case ofthe optical disk of this embodiment, an angle A1 of the side wall of thetrack groove of a first information storage layer 1001 is different froman angle A2 of the side wall of the track groove of a second informationstorage layer 1002.

The signal of the track groove detected at the optical head is obtainedwhen the reflected light of the track groove interferes with the lightrefracted by the track groove on the photo-detection area of the opticalhead. When the angle of the side wall of the track groove increases,this interference state changes, consequently having the effectequivalent to the effect produced from the shallower track groove.

Assuming that the angle A1 of the side wall of the track groove=theangle A1 of the side wall of the track groove, the proportion ofintensity of the light reflected by the first information storage layer1001 and detected by the photo-detection area to the intensity of thelight incident from the optical head was approximately 7%, whereas theproportion of intensity of the light reflected by the second informationstorage layer 1002 and detected by the photo-detection area wasapproximately 5%.

Assuming that the depth of the track groove in the respectiveinformation storage layers is 17 nm, a CN ratio when the optical headwith a numerical aperture of 0.85 that emits a laser beam with awavelength of 405 nm is used to perform recording/reproduction is shownin Table 3. The required CN ratio is 30 dB or greater.

TABLE 3 Angle 45° Angle 60° 1st information storage 38 dB 33 dB layer(A1) 2nd information storage 33 dB — layer (A2)

As is apparent from Table 3, by making the slope angle A1 of the sidewall of the track groove of the first information storage layer 1001 byapproximately 15 degrees greater than the slope angle A2 of the sidewall of the track groove of the second information storage layer 1002,it was possible to obtain almost the same CN ratio (33 dB) as that ofthe readout signal from the second information storage layer 1002. Thus,almost the same CN ratio of 30 dB or more was obtained for the readoutsignals from both information storage layers.

When the amount of light reflected by the information storage layer anddetected by the photo-detection area of the optical head variesconsiderably depending on the information storage layer, a problemoccurs when the focus of the optical head is moved between the twoinformation storage layers when recording/reproduction is performed ondifferent information storage layers. To avoid this problem, it ispreferable to adjust the amount of light corresponding to theinformation storage layer whose amount of light detected becomes amaximum to not more than two times the amount of light corresponding tothe information storage layer whose amount of light detected becomes aminimum.

Then, a method of producing a multilayer optical disk according to thisembodiment will be explained. This method of producing a multilayeroptical disk is substantially the same as the method of producing amultilayer optical disk according to the first embodiment. However, thisembodiment heats the master disk at a temperature close to the meltingpoint of the photosensitive material (e.g., approximately 120° C.) forseveral minutes between the step of exposure and development and thestep of plating on the photoresist. In this heating step, the surface ofthe photoresist is soften and the surface region of the photoresist isrounded by surface tension. Adjusting this temperature of heatingprocessing and/or time of heating processing allows the angle of theside wall of the track groove of the master disk to be controlled.

Even after producing the stamper, exposing the stamper to the plasma ofan Ar gas allows the angle of the side wall of the track groove to bechanged. This is because the plasma processing allows an electric fieldto concentrate on the corners of the track groove and allows the cornersto be sputtered ahead of other parts and rounded. The variation in theshape of the corners through this processing depends on the time ofplasma processing and state of plasma (ion density and ion irradiationenergy). Thus, changing the time of plasma processing and power appliedused for plasma generation from one stamper to another also makes itpossible to produce a multilayer optical disk according to thisembodiment. Furthermore, it is also possible to generate plasma usinganother gas (e.g., oxygen gas) instead of the above-described Ar gas orin addition to the Ar gas.

In the aforementioned embodiments, shape parameters such as theamplitude of wobbling of the track groove, the depth and the side wallangle of the track groove are adjusted so as to reduce noise of thesecond information storage layer. However, noise of the firstinformation storage layer may also increase depending on the structureor method of producing a multilayer optical disk. In such a case, it isalso possible to adjust the shape parameters of the track groove so asto reduce noise of the first information storage layer. Moreover, it ispossible not only to wobble both sides of the track groove but also towobble each side independently or wobble only one side.

Embodiment 4

The wobbling of a track groove need not be constructed of sine waveformsalone. Part of the wobbling can also be changed to a rectangularwaveform as shown in FIG. 13. Giving features distinguished from a sinewaveform to the track groove allows information other than clockinformation (sub-information such as address information) to be recordedin the track groove. Making the amount of amplitude of the rectangularwaveform greater than the amount of amplitude of the sine waveformallows the sub-information to be detected with high quality.

In order to give the shapes as shown in FIG. 13 to the track groove, itis possible to expose the above-described photoresist to light using adeflector (e.g., deflector using electro-optical effects) that allows alaser beam to be deflected in a frequency band not smaller than 10 timesa sine waveform frequency.

Hereunder, the optical disk specified by a combination of displacementpatterns of a plurality of types of track groove wobbling structure willbe explained in detail with reference to the drawings.

The surface shape of the track groove according to this embodiment notonly comprises simple sine waveforms as shown in FIG. 3 or FIG. 5 alone,but also has a portion in the shape different from a sine waveform atleast partially. The basic configuration of such a wobbled groove isdisclosed in the Specifications of the patent applications filed by thepresent applicant (Japanese Patent Application No. 2000-6593, JapanesePatent Application No. 2000-187259 and Japanese Patent Application No.2000-319009).

Here, FIG. 14( a) and FIG. 14( b) will be referenced. FIG. 14( a)illustrates the four types of basic elements that make up a wobblepattern of the track groove 2. In FIG. 14( a), smooth sine waveformportions 100 and 101, a rectangular portion 102 with a steepradialy-outward displacement and a rectangular portion 103 with a steepradially-inward displacement are shown. By combining these elements orportions with each other, the four types of wobble patterns 104 through107 shown in FIG. 14( b) are formed.

The wobble pattern 104 is a sine wave with no rectangular portions. Thispattern will be herein referred to as a “fundamental waveform”. Itshould be noted that the “sine wave” is not herein limited to a perfectsine curve, but may broadly refer to any smooth wobble.

The wobble pattern 105 includes portions that are displaced toward thedisk outer periphery more steeply than the sine waveform displacement.Such portions will be herein referred to as “radialy-outward displacedrectangular portions”.

In an actual optical disk, it is difficult to realize the displacementof a track groove in the disk radial direction vertically to thetracking direction. Accordingly, an edge actually formed is notperfectly rectangular. Thus, in an actual optical disk, an edge of arectangular portion may be displaced relatively steeply compared to asine waveform portion and does not have to be perfectly rectangular. Ascan also be seen from FIG. 14( b), at a sine waveform portion, adisplacement from the innermost periphery toward the outermost peripheryis completed in a half wobble period. As for a rectangular portion, asimilar displacement may be finished in a quarter or less of one wobbleperiod, for example. Then, the difference between these shapes is easilydistinguishable.

It should be noted that the wobble pattern 106 is characterized byradially-inward displaced rectangles while the wobble pattern 107 ischaracterized by both “radially-inward displaced rectangles” and“radially-outward displaced rectangles”.

The wobble pattern 104 consists of the fundamental waveform alone.Accordingly, the frequency components thereof are defined by a“fundamental frequency” that is proportional to the inverse number ofthe wobble period T. In contrast, the frequency components of the otherwobble patterns 105 through 107 include not only the fundamentalfrequency components but also high-frequency components. Thosehigh-frequency components are generated by the steep displacements atthe rectangular portions of the wobble patterns.

If the coordinate system shown in FIG. 3 is adopted for each of thesewobble patterns 105 through 107 to represent the Y coordinate of aposition on the track centerline by a function of the X coordinatethereof, then the function may be expanded into Fourier series. Theexpanded Fourier series will include a term of a sin function having anoscillation period shorter than that of sin (2πx/T), i.e., a harmoniccomponent. However, each of these wobble patterns includes a fundamentalwave component. The frequency of the fundamental waveform will be hereinreferred to as a “wobble frequency”. The four types of wobble patternsdescribed above have a common wobble frequency.

In the present invention, instead of writing address information on thetrack groove 2 by modulating the wobble frequency, the multiple types ofwobble patterns are combined with each other, thereby recording varioustypes of information, including the address information, on the trackgroove. More specifically, by allocating one of the four types of wobblepatterns 104 through 107 to each predetermined section of the trackgroove, four types of codes (e.g., “B”, “S”, “0” and “1”, where “B”denotes block information, “S” denotes synchronization information and acombination of zeros and ones represents an address number or an errordetection code thereof may be recorded.

Next, the fundamentals of an inventive method for reading information,which has been recorded by the wobble of the track groove, from theoptical disk will be described with reference to FIGS. 15 and 16.

First, FIGS. 15 and 16 will be referred to.

FIG. 15 illustrates a main portion of a reproducing apparatus, whileFIG. 16 illustrates a relationship between the track groove and a readsignal.

The track groove 1200 schematically illustrated in FIG. 16 is scanned bya read laser beam 1201 so that the spot thereof moves in the arroweddirection. The laser beam 1201 is reflected from the optical disk toform reflected light 1202, which is received at detectors 1203 and 1204of the reproducing apparatus shown in FIG. 15. The detectors 1203 and1204 are spaced apart from each other in a direction corresponding tothe disk radial direction and each output a voltage corresponding to theintensity of the light received. If the position at which the detectors1203 and 1204 are irradiated with the reflected light 1202 (i.e., theposition at which the light is received) shifts toward one of thedetectors 1203 and 1204 with respect to the centerline that separatesthe detectors 1203 and 1204 from each other, then a difference iscreated between the outputs of the detectors 1203 and 1204 (which is“differential push-pull detection”). The outputs of the detectors 1203and 1204 are input to a differential circuit 1205, where a subtractionis carried out on them. As a result, a signal corresponding to thewobble shape of the groove 1200 (i.e., a wobble signal 1206) isobtained. The wobble signal 1206 is input to, and differentiated by, ahigh-pass filter (HPF) 1207. As a result, the smooth fundamentalcomponents that have been included in the wobble signal 1206 areattenuated and instead a pulse signal 1208, including pulse componentscorresponding to rectangular portions with steeps gradients, isobtained. As can be seen from FIG. 16, the polarity of each pulse in thepulse signal 1208 depends on the direction of its associated steepdisplacement of the groove 1200. Accordingly, the wobble pattern of thegroove 1200 is identifiable by the pulse signal 1208.

Next, referring to FIG. 17, illustrated is an exemplary circuitconfiguration for generating the pulse signal 1208 and a clock signal1209 from the wobble signal 1206 shown in FIG. 16.

In the exemplary configuration illustrated in FIG. 17, the wobble signal1206 is input to first and second band-pass filters BPF1 and BPF2, whichgenerate the pulse and clock signals 1208 and 1209, respectively.

Supposing the wobble frequency of the track is fw (Hz), the firstband-pass filter BPF1 may be a filter having such a characteristic thatthe gain (i.e., transmittance) thereof reaches its peak at a frequencyof 4 fw to 6 fw (e.g., 5 fw). In a filter like this, the gain thereofpreferably increases at a rate of 20 dB/dec, for example, in a rangefrom low frequencies to the peak frequency, and then decreases steeply(e.g., at a rate of 60 dB/dec) in a frequency band exceeding the peakfrequency. In this manner, the first band-pass filter BPF1 canappropriately generate the pulse signal 1208, representing therectangularly changing portions of the track wobble, from the wobblesignal 1206.

On the other hand, the second band-pass filter BPF2 has such a filteringcharacteristic that the gain thereof is high in a predeterminedfrequency band (e.g., in a band ranging from 0.5 fw to 1.5 fw andincluding the wobble frequency fw at the center) but is small at theother frequencies. The second band-pass filter BPF2 like this cangenerate a sine wave signal, having a frequency corresponding to thewobble frequency of the track, as the clock signal 1209.

The track groove in this embodiment has the aforementioned complicatedwobble shape, and therefore if the CN ratio of the signal reproducedfrom one of the two information storage layers is reduced compared tothe CN ratio of the signal reproduced from the other, neither a clocksignal nor address information, etc., may be reproduced accurately.

Thus, as explained with respect to Embodiments 1 to 3, by adjusting theshape factors of the track groove layer by layer, it is possible tooptimize the CN ratio of the readout signal.

The present invention is not limited to a multilayer optical disk havingtwo information storage layers, but is also applicable to a multilayeroptical disk having three or more information storage layers.Furthermore, the shape factors of the track groove are not limited tothe aforementioned elements and the method of changing each shape factorfor each information storage layer is not limited to the aforementionedmethod, either. Moreover, the combination of a plurality of shapefactors explained in Embodiments 1 to 3 can also be changed for eachinformation storage layer.

INDUSTRIAL APPLICABILITY

The present invention provides a type of multilayer optical disk whichrecords information in the wobbling of track grooves, changes elementsof changing the amplitude of a signal with respect to the degree ofmodulation of the signal according to the wobbling of the track groove,that is, the amplitude of the signal with respect to the amount ofreflected light (shape factors of the track groove) for each informationstorage layer, and can thereby read the information recorded accordingto the wobbling of the track groove at a satisfactory CN ratio.

1. A multilayer optical disk comprising: a first information layerincluding a first wobbling track groove having a first wobblingamplitude along the radial direction of the disk and a first depth; anda second information layer located further from a light incident surfaceof the multilayer optical disk than the first information layer, thesecond information layer including a second wobbling track groove havinga second wobbling amplitude along the radial direction of the multilayeroptical disk and a second depth; wherein the first wobbling amplitude isdifferent from the second wobbling amplitude and the first depth isdifferent from the second depth.
 2. A multilayer optical disk accordingto claim 1, wherein at least one of the first wobbling track groove andthe second wobbling track groove contains a fundamental frequencycomponent defining a clock signal.
 3. A multilayer optical diskaccording to claim 2, wherein at least one of the first wobbling trackgroove and the second wobbling track groove exhibits a shape that variesaccording to sub-information and contains a higher frequency componentthan the fundamental frequency component.
 4. A multilayer optical diskaccording to claim 3, wherein the sub-information contains positionalinformation indicating addresses on the multilayer optical disk.
 5. Amultilayer optical disk according to claim 1, wherein the secondwobbling amplitude is larger than the first wobbling amplitude.
 6. Amultilayer optical disk according to claim 5, wherein the second depthis larger than the first depth.
 7. A multilayer optical disk accordingto claim 1, wherein a wobbling shape of at least one of the firstwobbling track groove and the second wobbling track groove includes acombination of a sine wave and a rectangular waveform.
 8. A multilayeroptical disk according to claim 7, wherein an amplitude of therectangular waveform is greater than an amplitude of the sine wave.
 9. Amultilayer optical disk according to claim 1, wherein the second depthis larger than the first depth.
 10. A multilayer optical disk accordingto claim 1, wherein at least one of the first wobbling track groove andthe second wobbling track groove is formed by combining a plurality ofwobble patterns each having a same fundamental frequency.
 11. Amultilayer optical disk comprising: a first information layer includinga first wobbling track groove having a first wobbling amplitude alongthe radial direction of the disk; and a second information layer locatedfurther from a light incident surface of the multilayer optical diskthan the first information layer, the second information layer includinga second wobbling track groove having a second wobbling amplitude alongthe radial direction of the multilayer optical disk; wherein the secondwobbling amplitude is larger than the first wobbling amplitude.
 12. Amultilayer optical disk according to claim 11, wherein at least one ofthe first wobbling track groove and the second wobbling track groove isformed by combining a plurality of wobble patterns each having a samefundamental frequency.
 13. A multilayer optical disk comprising: a firstinformation layer including a first wobbling track groove having a firstdepth; and a second information layer located further from a lightincident surface of the multilayer optical disk than the firstinformation layer, the second information layer including a secondwobbling track groove having a second depth; wherein the second depth islarger than the first depth.
 14. A multilayer optical disk according toclaim 13, wherein at least one of the first wobbling track groove andthe second wobbling track groove is formed by combining a plurality ofwobble patterns each having a same fundamental frequency.
 15. Amultilayer optical disk comprising: a first information layer includinga first wobbling track groove having a first tilt angle of a first sidewall; and a second information layer located further from a lightincident surface of the multilayer optical disk than the firstinformation layer, the second information layer including a secondwobbling track groove having a second tilt angle of a second side wall;wherein the second tilt angle of the second side wall of the secondwobbling track groove is smaller than the first tilt angle of the firstside wall of the first wobbling track groove.
 16. A multilayer opticaldisk according to claim 15, wherein at least one of the first wobblingtrack groove and the second wobbling track groove is formed by combininga plurality of wobble patterns each having a same fundamental frequency.